4781 Chap10 11

Chapters 10 & 11: 

Chapters 10 & 11 Extending Networks (Repeaters, Bridges, Switches)

Motivation: 

Motivation Recall LAN technologies have their advantages for cost, speed, and distance Example: CSMA/CD cannot work across arbitrary distance However Users desire arbitrary distance connections Example: two computers across a corporate campus are part of one workgroup

Extension Techniques: 

Extension Techniques Must not violate design assumptions Often part of original design Example technique Repeater Bridge Router Gateway …

Illustration Of ExtensionFor One Computer: 

Illustration Of Extension For One Computer Optical fiber Has low delay Has high bandwidth

Repeater: 

Repeater Hardware device Connects two LAN segments Copies signal from one segment to the other Amplifies signal Maximum of 4 repeaters allowed

Repeater (continued): 

Repeater (continued) Amplifies signals from one segment and sends to the other Operates in two directions simultaneously DRAWBACK: Forwards noise and collisions

Repeaters and the OriginalEthernet Wiring Scheme: 

Repeaters and the Original Ethernet Wiring Scheme Designed for office Only two repeaters between any pair of stations

Hub: 

Hub Physically Small electronic device Has connections from several computers (e.g., 4 or 20) Logically Operates on signals Propagates each incoming signal to all connections Similar to connecting segments with repeaters Does not understand packets Extremely low cost

Connection Multiplexing: 

Connection Multiplexing Concept Multiple stations share one network connection Motivation Cost Convenience of wiring Hardware device required

Illustration of Connection Multiplexing: 

Illustration of Connection Multiplexing Multiplexing device attached to network Stations attach to device Predates hubs

Modern Equivalent OfConnection Multiplexing: 

Modern Equivalent Of Connection Multiplexing Hubs used now Connections on a hub One for each attached computer One for another hub Multiple hubs Can be interconnected in a daisy chain Operate as one giant hub Called stacking

Slide12: 

Bridges A Bridge C LANs can be connected by devices called bridges, which operate in the data link layer. Bridges do not examine the network layer header.

Bridge: 

Bridge Hardware device Connects two LAN segments Forwards frames Does not forward noise or collisions Learns addresses and filters Allows independent transmission

Bridge Algorithm: 

Bridge Algorithm Listen in promiscuous mode Watch source address in incoming frames Make list of computers on each segment Only forward if necessary Always forward broadcast / multicast

Illustration of a Bridge: 

Illustration of a Bridge Bridge uses source address to learn location of computers Learning is completely automated

Illustration of a Bridge: 

Illustration of a Bridge When a frame arrives, a bridge must decide whether to discard or forward it, and if the latter, on which LAN to put the frame. The decision is made by looking up the destination address in a big (hash) table inside the bridge.

Illustration of a Bridge: 

Illustration of a Bridge Transparent Bridges When the bridges are first plugged in, all the hash tables are empty. None of the bridges know where any of the destinations are, so they use the flooding algorithm. Use backward learning algorithm. If a frame comes from LAN1 with source address A, the bridge learns that host A is in LAN1. The topology can change as machines and bridges are powered up and down and moved around. To handle dynamic topologies, whenever a hash table entry is updated, the time is recorded. Periodically, a process in the bridge scans the hash table and purges all entries that are old.

Illustration of a Bridge: 

Illustration of a Bridge Routing procedure for a transparent bridge: 1. If the destination and source LANs are the same, discard the frame. 2. If the destination and source LANs are different, forward the frame. 3. If the destination LAN is unknown, use flooding.

Extending a Bridge: 

Extending a Bridge Typically optical fiber Can span buildings

Satellite Bridging: 

Satellite Bridging Can span arbitrary distance

Apparent Problem: 

Apparent Problem Complex bridge connections may not be apparent Adding one more bridge inadvertently introduces a cycle Consider a broadcast frame

Apparent Problem: 

Apparent Problem Avoid looping in parallel bridges.

Spanning Tree Algorithm: 

Spanning Tree Algorithm Allows cycles Used by all bridges to Discover one another Break cycles(s) Known as Distributed Spanning Tree (DST)

Spanning Tree Algorithm: 

Spanning Tree Algorithm Spanning Tree Bridges

Spanning Tree Algorithm: 

Spanning Tree Algorithm Spanning Tree Bridges

Spanning Tree Algorithm: 

Spanning Tree Algorithm Spanning Tree Bridges To build the spanning tree, first the bridges have to choose one bridge to be the root of the tree. They make this choice by having each one broadcast its serial number, installed by the manufacturer, and guaranteed to be unique worldwide. The bridge with the lowest serial number becomes the root. Next, a tree of shortest paths from the root to every bridge and LAN is constructed.

Switch: 

Switch Electronic device Physically similar to a hub Logically similar to a bridge Operates on packets Understands addresses Only forwards when necessary Permits separate pairs of computers to communicate at the same time Higher cost than hub

Conceptual Switch Function: 

Conceptual Switch Function Conceptual operation One LAN segment per host Bridge interconnects each pair of segments Underlying topology can be thought of as a complete graph